The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Due to regulation of volatile organic solvent emissions, there is a continuing desire to reduce the volatile organic content of coating compositions. Solvents, which may be expensive, evaporate during application and curing. Therefore, coatings known in the art as “high solids” are often used to limit the cost and amount of volatile solvents vented to the atmosphere.
A high solids level can result in an undesirable viscosity, limiting the methods of application. A high solids coating of suitable viscosity can sometimes be achieved through use of low molecular weight solution polymers. However, a problem associated with these high solids coatings is that at approximately a 50-60% solids level the coating can exhibit significant sagging. Industry generally requires a 2.5 mil topcoat thickness without sagging, and thixotropes are often added to achieve the required coating thickness without sagging. Thixotropes are a further added expense and can result in coatings having an undesirable appearance, for example lower gloss.
In contrast, the nonaqueous dispersion (NAD) approach to high solids clearcoats offers advantages over conventional solution based clearcoats A high solids coating can be achieved with a nonaqueous dispersion, while maintaining or increasing the molecular weight, because the polymers of the coating are in a dispersed phase. For instance, during application of the nonaqueous dispersion the viscosity is low because the polymer is in a dispersed phase. After application and solvent evaporation, there is a high viscosity since the molecular weight is not lowered. This high viscosity and the pseudoplastic character of the NAD provide resistance to sag. Thus, nonaqueous dispersions break the viscosity dichotomy required of good coatings, i.e., a low viscosity during application and a high viscosity after application.
Although nonaqueous dispersion resins generally show superior resistance to cratering and solvent pop, there remains an additional need to improve environmental etching and scratch and mar resistance. Environmental etching manifests as spots or mark on or in the clearcoat. The high degree of clarity exhibited by clearcoats generally makes it easier to observe such defects. General appearance and ease of repair also remain as a concern in the industry.
Accordingly, there is a persistent search for high solids clearcoat compositions that have excellent appearance, and improved durability, hardness, flexibility, sag resistance, crack and chip resistance, scratch and mar resistance, resistance to environmental etch, and ease of repair in comparison to conventional clearcoats.